No, you're not imagining it. Soil in potted plants really does shrink over time. Here's where the soil goes, and the one thing most houseplant hobbyists do, that you should NEVER do when faced with shrinking soil...
Just because your soil level is so low you need to abseil down from the edge of the pot to reach it, does NOT mean you need a panic repot. It doesn't mean your plant is rootbound either.
This is what's going on, where the soil's gone, what to do to fix it, and how to decide when you should repot vs just give your plant a safer soil lift instead.
Soil shrinkage is natural, but that doesn't mean it's good. If your soil level is more than a couple of cms down from the top edge of your pot, the risk starts to go up of a long list of common, avoidable plant problems.
But NEVER simply add more soil to the top! That's the most natural thing to do, but could cause some of those same problems you're trying to prevent!
Where'd the soil go?
It's normal for potting mix to reduce over time, with the level dropping further and further down inside the pot.
Although you can lose a little out the drainage holes when you water, especially straight after you repot into fresh potting mix, that doesn't account for where most of it's gone.
Even a fine, dense soil is a good 20% to 30% air. That's the sort of mix you'd put moisture-lovers indoors, and many outdoor plants in, but NOT most houseplants.
Instead, most of our indoor friends tend to do better in a chunkier potting mix with a lot more air, like your typical bark-based, soil-free aroid mix.
Soil naturally breaks down, settles and compacts over time. As that air space reduces, the level visibly goes down in the pot. That can be bad news for houseplants.
Some of it also goes thanks to enzymes and microbes, like bacteria and fungi, that live in healthy soil. Those beneficials get busy doing their job, breaking down the organic matter in potting mix over time, turning it into nutrients plants use to grow. Although I'd argue that's a more common cause outdoors than in the relatively sterile potting mix most indoor plants grow in.
What are the risks of soil shrinkage?
Watch out for root rot, powdery mildew, stem rot, bald stems, pest infestations, stunted growth, yellowing leaves and leaf drop. Or just fix that soil level before those problems come knocking.
As the percent of air in the pot reduces, the risk of root rot increases. Less air means the potting mix might now hold more water. When potting mix stays wet for longer, it's more likely to attract soil pests like fungus gnats.
As the level of the soil shrinks further and further down inside the pot, airflow reduces. The top of the potting mix takes longer to dry out, increasing the risk of stem rot. Plus powdery mildew and other fungi typically love moist, lower airflow conditions too.
Plus some plants - like many 'string of' and 'chain of' varieties - benefit from a combo of light and airflow on the base of the plant. If they end up down in the dark as the soil level shrinks, struggling to get the light and air they need, you can end up with bald, bare stems.
What should you do instead of repotting?
I'm probably the opposite of a lot of houseplant hobbyists, but I tend NOT to repot unless absolutely necessary.
Not just because I not-so-secretly dislike the hassle and mess of repotting, but also because it can do more harm than good.
Just because soil levels are low is not always a good enough reason to put a plant through the stress and shock of repotting in my books.
Give your plants a soil lift instead
Unless there are other good reasons to repot, I give my plants a simple soil lift instead. It's generally best not to just dump fresh soil on TOP of the existing potting mix. That can also cause problems like stem rot by burying parts of your plant that should be above-ground.
But what you can do is gently slide your plant out of its pot, taking care not to disturb the roots, and add fresh potting mix to the BOTTOM of the pot instead. Then gently sliding your plant back in on top.
This lifts that soil level back up, reducing the risk of all those common plant problems, while avoiding transplant shock. Win win!
When should you fully repot?
That's not to say I never repot. Come spring and summer I enevitably have a long list of plants to repot every year.
I do also repot in winter if needs must, but I 'cheat' with heat mats, higher humidity and grow lights on hand for recovery (I find temperature is the biggie for recovery).
- I do repot when what's in the pot is holding too much water for too long, provided the plant dislikes 'wet feet' (but I try giving the plant more light first).
- I do repot when a plant is well and truly rootbound.
- I do repot when I discover root rot unless it's minor and manageable.
- I do repot if the potting mix is no longer able to retain water for long enough (for me I judge that by how often I have to water, if it's more than once a week I repot).
And yes, if the potting mix has broken down and shrunk so much that it's no longer serving its purpose, or the plant is on struggle street, then yes, that plant gets added to my repotting list too.
Otherwise you'll catch me quickly giving my plants a soil lift instead 😊
Happy growing,
Anna 🪴💚
PS: Time to repot? The one simple step you do BEFORE you repot can make all the difference... Find out here >
ABOUT THE AUTHOR: Anna is the owner of plant store Love That Leaf which specialises in products and advice for indoor plants. She is also the resident indoor plants writer for NZ Gardener magazine. Her houseplant care articles appear in print in magazines in-stores NZ-wide, and online on Stuff, The Post, NineHoney and more.